1. Field of the Invention
The present invention relates to a measuring instrument used in industries to detect locational or angular information.
2. Related Background Art
The applicant has been making efforts to develop what is called a grating interference-type encoder for detecting variations in the position or speed of an object by applying the diffraction interference phenomenon of light.
In particular, the applicant has realized an encoder of a much higher resolution than geometrical optical encoders by employing a fine scale on the order of xcexcm to obtain two diffracted beams, and causing them to interfere with each other.
This grating interference type encoder synthesizes the wave surfaces of the two diffracted beams to generate an interference pattern.
As in interference length measuring machines, however, a difference in polarization is established between the two beams and used to generate a plurality of phase difference signals, that is, an A phase signal and a B phase signal.
In general, light detecting elements such as a wavelength plate, a non-polarization or diffraction grating beam splitter, and a polarization plate or prism are arranged in a space so that corresponding light-receiving elements can detect emitted beams.
In the above described conventional example, however, the arrangement of optical parts for generating the phase difference signals is complicated, and the light-receiving elements are arranged in the space so as to correspond to the different signals. Accordingly, a large space is required, limiting the reduction of the size.
Further, four-piece photo diodes or the like may be used to make the electrical characteristics of the light-receiving elements uniform and to allow users to is use the encoder easily.
In this case, optical parts such as prisms of several mm size must be properly joined together so as to illuminate the light-receiving elements in parallel. Thus, expensive photodiodes having a light-receiving surface of several mm size at minimum must be used. With this method, the light-receiving elements have a large capacity and thus fail to provide a fast response performance.
It is an object of the present invention to solve the above problems to provide a measuring instrument having an optical system of a reduced size and an optical element that allows the size of the optical system to be reduced.